1. Field of the Invention
The present invention relates to high pressure discharge lamps used in devices such as data projectors, liquid crystal projectors or DLP (digital light processor) projectors. The invention relates in particular to high pressure discharge lamps wherein mercury is enclosed in the light emission part in an amount of at least 0.15 mg/mm3 and the mercury vapor pressure becomes at least 110 atm.
2. Description of Related Art
Recently, liquid crystal projectors or DLP projectors using the digital light processing technology are becoming widespread. As the light source for the image projection, short arc type high pressure discharge lamps are used.
FIG. 5 is a partially sectional view showing the configuration of a high pressure discharge lamp shown in JP 2004-363014 A and corresponding to US 2005-003729 A1. The high pressure discharge lamp 10 is configured such that it comprises a discharge vessel consisting of a spherical light emission part 4 formed in the center portion and sealing parts 5 formed at both ends of said light emission part 4. In the interior of the light emission part 4, a pair of electrodes 1 is arranged, and core rods 7 of the electrodes 1 and metal foils 6 connected to the core rods 7 are buried and sealed air-tightly in the sealing parts 5. With such a high pressure discharge lamp 10, the expansion of the arc is suppressed by rendering the mercury vapor pressure at the time of switching-on high, and a further increase of the light output can be expected.
The core rod 7 of the electrode 1 and the metal foil 6 are joined by a laser. For example, laser light is irradiated from the metal foil 6 side, after the core rod 7 and the metal foil 6 have been closely contacted, and both are joined by melting both the molybdenum (Mo) constituting the metal foil 6 and the tungsten (W) constituting the core rod 7. As it is possible to fuse the core rod 7 and the metal foil 6 together, when performing the joining using a laser, the reliability of the electric connection can be improved and the joining strength can be increased.
With this kind of lamp, the core rod 7 of the electrode 1 and the metal foil 6 expand thermally at the time of switching-on, while at the time of switching-off a contraction occurs because of the cooling. The core rod 7 adheres well to the glass constituting the sealing part 5 and hardly expands and contracts, but the metal foil 6 easily expands and contracts as the glass does not adhere to it as well as to the core rod 7. Therefore, the core rod 7 hardly expands and contracts thermally while the metal foil 6 easily expands and contracts thermally at the times of switching-on and switching-off of the high pressure discharge lamp 10.
When the switching-on and switching-off of the high pressure discharge lamp is performed repeatedly while keeping the different amounts of thermal expansion and contraction between the core rod 7 and the metal foil 6 as they are, the thickness of the portion of the metal foil 6 overlapping the core rod 7 becomes thin and the electric load acting on the metal foil 6 becomes high. From experience, a bending of the end of the core rod at the metal foil 6 side is also known. The metal foil 6 is heated by the increase of the electric load and is, in addition, pressed by the bending of the core rod 7 and fuses.